The method for welding by a magnetically driven arc is known as a method for butt-welding two objects of welding such as metal plates or metal pipes. This method for welding by a magnetically driven arc comprises the steps of placing two objects of welding in butt formation with a small gap between the end edges thereof, moving an arc produced in said small gap along the welding line under the effect of an electromagnetic force, heating said end edges by the heat of said arc to a welding temperature, and then, pushing said heated end edges of said two objects of welding against each other to weld together said two objects of welding.
In the conventional method for welding by a magnetically driven arc mentioned above, however, the arc produced in the small gap between the two objects of welding does not move in the thickness direction of the end edges of the two objects of welding. When the two objects of welding have a large thickness, therefore, it is not possible by this method to uniformly heat the end edges in the thickness direction, thus leading to an unsatisfactory weld.
The following methods are known as methods for solving the above-mentioned problem:
(1) A method for controlling a magnetically driven arc, disclosed in Japanese Patent Provisional Publication No. 123,542/79 dated Sept. 25, 1979, which comprises:
placing two objects of welding in butt formation with a small gap between the end edges thereof, said small gap forming a welding line between said two objects of welding; connecting said two objects of welding to a welding power source by a cable to form an electric circuit comprising said welding power source, said cable and said two objects of welding; flowing a welding current through said electric circuit to produce an arc in said small gap between said two objects of welding; providing a magnetizing coil on each of the surface side and the back side of said two objects of welding in parallel with said welding line between said two objects of welding, near said end edge of each of said two objects of welding, said magnetizing coils producing a magnetic flux in said small gap in the thickness direction of said two objects of welding; moving said arc along said welding line under the effect of electromagnetic force in the direction of said welding line produced by said magnetic flux and said welding current flowing through said arc; and, at the same time, changing the ratio of exciting current between the magnetizing coil provided on the surface side of each of said two objects of welding and the magnetizing coil provided on the back side of each of said two objects of welding to position said arc at the center of the end edges of said two objects of welding in the thickness direction thereof (hereinafter referred to as the "prior art (1)"). (2) A method and an apparatus for rotary arc welding disclosed in Japanese Patent Publication No. 32,851/77 dated Aug. 24, 1977, which comprise:
placing two metal pipes in butt formation with a small gap between the end edges thereof, said small gap forming a welding line between said two metal pipes; connecting said two metal pipes to a welding power source by a cable to form an electric circuit comprising said welding power source, said cable and said two metal pipes; flowing a welding current through said electric circuit to produce an arc in said small gap between said two metal pipes; providing a magnetizing coil on the surface side of each of said two metal pipes in parallel with said welding line between said two metal pipes, near said end edge of each of said two metal pipes, said magnetizing coils producing a magnetic flux in said small gap in the thickness direction of said two metal pipes; moving said arc along said welding line under the effect of an electromagnetic force in the direction of said welding line produced by said magnetic flux and said welding current flowing through said arc; and providing an electrically conductive cylinder connected to another power source in said two metal pipes concentrically with said two metal pipes at a position near the small gap between said end edges of said two metal pipes, to continuously and controllably moving said arc in the thickness direction of the end edges of said two metal pipes under the effect of magnetic force produced by said electrically conductive cylinder (hereinafter referred to as the "prior art (2)").
In the above-mentioned prior art (1), however, it is necessary to provide another magnetizing coil on the back side of each of the two objects of welding for the purpose of positioning the arc produced in the small gap between the two objects of welding at the center of the end edges of the two objects of welding in the thickness direction thereof. In the above-mentioned prior art (2), it is necessary to provide an electrically conductive cylinder connected to another power source in the two metal pipes concentrically with the two pipes at a position near the small gap between the end edges of the two pipes for the purpose of continuously and controllably moving the arc produced in the small gap between the two pipes in the thickness direction of the end edges of the two pipes. Therefore, both the prior art (1) and the prior art (2) require a complicated welding apparatus, and when two pipes are to be welded, it is not easy to set another magnetizing coil or an electrically conductive cylinder in the two pipes. Particularly when welding a new pipe to an already laid pipe in a laying site of pipes, it is difficult to install an electrically conductive cylinder, as in the above-mentioned prior art (2), connected to another power source in the two pipes concentrically with the two pipes at a position near the small gap between the end edges of the two pipes, to form another electric circuit.
Under such circumstances, there is a strong demand for an improved method which allows, when welding two objects of welding placed in butt formation with a small gap between the end edges thereof by a magnetically driven arc, uniform heating of the end edges of the two objects of welding in the welding line direction and in the thickness direction thereof to a welding temperature, by moving the arc produced in the small gap between the two objects of welding along the welding line formed between the two objects of welding, and, at the same time, by continuously moving the arc in the thickness direction of the end edges of the two objects of welding, thus which gives a high-quality weld, but such a method is not as yet proposed.